1. Field of the Invention
The present invention relates to a water base ink set for ink-jet recording and an ink-jet recording apparatus based on the use of the same.
2. Description of the Related Art
In the ink-jet recording system, ink droplets are formed by using the ink discharge method including, for example, the electrostatic attraction method, the method in which mechanical vibration or displacement is applied to the ink by using a piezoelectric element or the like, and the method in which bubbles are generated by heating the ink to utilize the pressure generated thereby. All or a part of the ink droplets are adhered to a recording objective material such as paper to perform the recording.
Those hitherto employed in many cases as the ink to be used when the recording is performed by using the ink-jet system include dye inks in which dyes such as direct dyes and acid dyes are used as coloring agents, for the following reason. That is, in the case of the dye ink, any inconvenience such as precipitation is hardly caused, and the handling of the ink is relatively easy, because the dye is dissolved in the ink. Further, in the case of the dye ink, the dye molecules absorb the light at a wavelength inherent in the dye molecules without allowing the dye molecules to cause any irregular reflection of the light on the surface and at the inside of the paper. Therefore, the dye ink is advantageous in that the printing can be performed with vivid coloration.
However, in the case of the dye ink, the dye is dissolved in the ink at the molecular level. Therefore, the dye behaves in the same manner as the solvent medium used for the ink after the printing. Therefore, the dye ink is disadvantageous in that the dye permeates into the paper and the concentration is lowered in the image area. Therefore, the pigment ink, which is based on the use of the pigment as the coloring agent, is presently used in many cases. In the case of the pigment ink, the pigment contained in the ink does not behave in the same manner as the ink solvent. The pigment particles are prohibited from movement, for example, by the additive and the fiber contained in the paper, and they do not permeate into the paper. Therefore, the amount of the pigment remaining on the paper surface is increased. As a result, it is possible to increase the concentration in the image area, and it is possible to enhance the contrast as well. The printer based on the ink-jet system involves a common problem, i.e., a problem of blurring in which the edge of the image area is notched due to the nonuniform spread of the ink. However, the pigment, which is dispersed as the solid content in the pigment ink, is prohibited from movement, for example, by the additive and the fiber contained in the paper, on the surface of the paper and/or at the inside of the paper. Therefore, the ink is scarcely blurred, and it is possible to obtain the sharp edge of the image area.
On the other hand, the pigment, which is dispersed in the pigment ink, has been disadvantageous in that the vividness of the printing is lowered, because the pigment particles irregularly reflect the light on the surface of the paper and/or at the inside of the paper. In view of the above, U.S. Pat. No. 5,745,140 discloses an ink set composed of a combination of a black ink based on the use of a black pigment such as carbon black as a coloring agent and a color ink based on the use of a dye as a coloring agent. When the printing is performed by using such an ink set, the following feature is obtained. That is, the letter edges are sharp, the contrast is high, and the visual recognition performance of the letters is excellent when letter data principally based on the use of the black color is subjected to the printing. When graphic data is subjected to the printing, color portions provide vivid coloration. Therefore, it is possible to satisfy both of the visual recognition performance of the letters and the vividness of the color portions.
However, as for the ink set composed of the pigment ink based on the use of the pigment as the coloring agent and the dye ink based on the use of the dye as the coloring agent, when the pigment ink and the dye ink make contact with each other and they are mixed with each other, then the pigment in the pigment ink, which is charged to have the negative electric charge, is bound to sodium ion as counter ion of the dye contained in the dye ink, and the electric charge of the negatively charged pigment becomes zero. Therefore, the electric repulsive force of the pigment is lost, the dispersion becomes unstable, and any coagulation occurs.
Japanese Patent Application Laid-open No. 2002-173623 discloses an ink set which is characterized in that a first electric charge (anionic property or cationic property) is given to a black ink, and a second electric charge (cationic property of anionic property), which is opposite to the first electric charge, is given to a color ink. In this known method, the property, which is opposite to the property possessed by the black ink, is given to the color ink as described above, and thus the qualities including, for example, the satisfactory bleeding quality, the quick drying performance, and the high optical density, are enhanced by coagulating the black pigment contained in the black ink when the black ink and the color ink make contact with each other on the paper surface. However, even in the case of the ink set based on the combination as described above, the clog-up occurs at the head nozzle of the printer due to the appearance of the pigment coagulate, and/or the pigment coagulate is adhered and secured to the surroundings of the head nozzle to damage the repelling ink coat surface. As a result, the discharge failure is caused, and printing quality is deteriorated. As described above, various inconveniences have hitherto occur due to the pigment coagulation.
A conventional ink-jet recording apparatus is disclosed in Japanese Laid-Open Patent Publication No. 2002-234151 corresponding to U.S. Pat. No. 6,631,974. The ink-jet recording apparatus comprises a printing head and a mechanism for collectively wiping nozzles for respective colors included in the printing head. In the case of the ink-jet recording apparatus as described above, the nozzle arrays for the respective colors, which are arranged in the printing head, are disposed extremely approximately to one another. For this reason, the following structure is provided. That is, a suction cap, which is equipped to suck and purge the ink contained in the printing head, collectively covers the nozzle arrays for two colors. In this structure, for example, the nozzles arrays for the black ink and the cyan ink are collectively covered with the suction cap. Therefore, when the suction purge is performed, for example, upon the initial introduction of the inks, then the inks of two colors, which are simultaneously sucked by the aid of the suction cap as described above, are mixed with each other in the suction cap, they thereafter arrive at a suction pump via a tube, and they are discharged to a drain tank.
In the case of the ink-jet recording apparatus as described above, the maintenance system is simplified, and it is possible to suppress the production cost to be cheap. However, for example, when the inks are initially introduced, then the pigment ink and the dye ink are mixed with each other, for example, at the printing head nozzle surface, the wiper, the suction cap, the suction pump, and the drain tank, and the pigment particles are coagulated. As a result, the printing head nozzle is clogged by the pigment coagulate, and/or the pigment coagulate is adhered and secured to the neighborhoods of the printing head nozzle to damage the repelling ink coat surface. Consequently, the discharge failure is caused, and the printing quality is deteriorated in some cases. Further, when the pigment coagulate is adhered and secured to the wiper and the suction cap, then the wiping operation is nonuniformly performed, and the air-tightness of the suction cap is deteriorated. Furthermore, another inconvenience also arises, for example, such that the pigment coagulate is secured to the interior of the suction pump to cause any malfunction. When the pigment particles are coagulated, the reliability of the ink-jet recording apparatus is extremely deteriorated.
In order to respond to the inconveniences as described above, for example, the following countermeasures are applied. That is, a plurality of wipers are arranged so that the pigment ink and the dye ink are not mixed with each other in the vicinity of the nozzles. The heads are provided in a divided manner for the pigment ink and the dye ink respectively. The maintenance system is divided into those to be used for the pigment ink and the dye ink. However, even when the countermeasures as described above are applied, a problem arises such that the production cost of the printer becomes expensive. Therefore, an ink set has been required, in which the pigment ink and the dye ink can be used in combination, the pigment is not coagulated, and it is possible to obtain a high printing quality even when the ink set is carried on a conventional low cost ink-jet recording apparatus.